1. Field of the Invention
The present invention relates to a semiconductor circuit, and particularly to a semiconductor circuit used in a CATV[CAble TeleVision] hybrid IC (HIC).
2. Description of the Related Art
To compensate for loss in connection cables in a HIC broadband amplifier for CATV, a gain slope is set by which gain of the amplifier rises with higher frequencies, but this gain slope fluctuates with variations in ambient temperature.
Fluctuations in the gain characteristic due to variations in ambient temperature must therefore be corrected to maintain a uniform signal level over the entire CATV system.
FIG. 1 is a circuit diagram showing the configuration of a circuit disclosed in Japanese Patent Laid-open No. 83910/82 that has been used in the prior art for compensating for fluctuations in gain characteristic caused by variations in ambient temperature.
As shown in FIG. 1, this example of the prior art is made up of: FET(Field Effect Transistor) 125 having its gate terminal connected to input terminal 121 by way of matching circuit 126, its drain terminal connected to output terminal 122 by way of matching circuit 127, and its source terminal grounded; inductor L121 having one terminal connected to input terminal 121; resistor R121 having one terminal connected to the terminal of inductor L121 that is not connected to input terminal 121 and its other terminal connected to gate bias supply terminal 123; thermistor R122 having one terminal connected to the terminal of inductor L121 that is not connected to input terminal 121 and its other terminal grounded; and inductor L122 having one terminal connected to output terminal 122 and its other terminal connected to drain bias supply terminal 124. The gate bias is supplied to the gate terminal of FET 125 from gate bias supply terminal 123 by way of matching circuit 126, inductor L121, and resistor R121, and the drain bias is supplied to the drain terminal of FET 125 from drain bias supply terminal 124 by way of matching circuit 127 and inductor L122.
In this case, the resistance of resistor R121 is set such that the relation of gate bias Vgs1 supplied from gate bias supply terminal 123 with respect to normal gate bias Vgs is: EQU .vertline.Vgs.vertline.&lt;.vertline.Vgs1.vertline.
When the ambient temperature is higher than room temperature, the resistance of thermistor R122 is smaller than for cases in which the ambient temperature is at room temperature due to its own temperature nonlinearity, and if the gate bias in such cases is Vgs2, then: EQU .vertline.Vgs2.vertline.&lt;.vertline.Vgs1.vertline.
Accordingly, assuming that the gain during gate bias Vgs1 is GVgs1, that the gain during gate bias Vgs2 is GVgs2, and that the gain is GVgs3 for an amplifier in which gate bias is set to Vgs in an ambient atmosphere that is at a higher temperature than room temperature, this amplifier having a bias circuit that is not provided with thermistor R122, then: EQU GVgs3&lt;GVgs2
and fluctuations in gain characteristic due to variations in ambient temperature can be compensated.
Nevertheless, the above-described circuit of the prior art has the following disadvantages:
Changing the FET frequency-gain characteristics by bias conditions means that temperature compensation is multiplied by bias, and bias voltage therefore reaches high levels in the case of high or low temperatures, increasing the current consumption of the FET and thereby increasing the heat stress imposed upon elements at high temperatures.
The operating points are determined based on the DC characteristic of elements, and the high frequency characteristic is therefore dependent on the DC characteristic, and differences in the DC characteristic of elements therefore cause discrepancies in the frequency characteristic.
FIG. 2 shows the fluctuation in the gain characteristic of the gain slope according to variations in ambient temperature.
As shown in FIG. 2, when ambient temperature varies in the case of a broad band such as in a CATV system, not only does the gain curve that represents the gain slope fluctuate parallel to the direction of gain, but the slope of the curve also varies.
Although compensation can be achieved for fluctuation parallel to the direction of gain in the circuit shown in FIG. 1, fluctuation for the slope cannot be compensated.
Because the bias is set by the voltage division ratio of the bleeder resistance, a plurality of elements are necessary for realizing temperature compensation, and this requirement both increases the scale of the circuit and raises costs.